In recent years, optical recording media using a laser as a light source have undergone marked development because of the possibility of high density recording, preservation and reproduction of information. An optical disc, included in such optical recording media, has a basic structure generally comprising a transparent substrate and a recording layer of various types coated thereon.
The transparent substrate for optical discs comprises in many cases a colorless transparent synthetic resin which typically includes a polycarbonate resin (hereinafter abbreviated as "PC") and a polymethyl methacrylate resin (hereinafter abbreviated as "PMMA"). Besides being superior in colorless transparency, these resins exhibit excellent properties of their own. However, they do not meet all the requirements for a substrate of optical discs, still leaving problems awaiting solution. For example, PC has a problem of high birefringence arising from its aromatic ring and also a problem of its water absorption or penetration properties, and PMMA has turned out to have disadvantages such as insufficient heat resistance, water absorption properties, and insufficient toughness.
The conventional resins have thus been employed while involving problems inherent therein. Besides, in actual use of the conventional resin substrate, different problems arise in relation to a recording layer formed thereon as hereinafter described.
A recording layer which can be used in optical discs has been extensively developed in conformity to end use. Known recording layers include a pit type exclusive for use in writing-reproduction called a write-once type, and a phase transition type utilizing a crystal transition phenomenon or an optomagnetic type utilizing an optomagnetic effect for use in writing-reproduction-erasure-rewriting called an erasable type. The recording layer of the write-once type generally comprises tellurium or an oxide or alloy compounds thereof, and that of the erasable type generally comprises amorphous alloy compounds of rare earth metals and transition metals, e.g., GdFe, TbFe, GdFeCo and TbFeCo, and other inorganic materials. These recording layers are formed on a transparent substrate usually by dry film formation techniques, such as cathode sputtering in a high degree of vacuum.
Water absorption and penetration properties of PC and PMMA cause not only warpage of the substrate per se due to swelling with moisture, but also corrosion of the recording layer due to water content having penetrated through the substrate, resulting in reduction of life of the optical disc.
Further, the recording layer of optical discs particularly of the write-once type and erasable type undergoes a rise in temperature to 200.degree. C or more on writing or erasing information. While this heat is not applied directly to the substrate, the substrate is likely to be heated to a considerably high temperature on writing or erasure. If a low heat-resistant resin is used as a substrate, the substrate or grooves of the substrate would undergo thermal deformation.
In the production of optical discs, a heat treatment is often conducted for the purpose of preventing changes of a substrate or a recording layer with a lapse of time. To increase productivity, it is desirable to conduct the heat treatment at a temperature as high as possible to reduce the treating time. From this point of view, use of a low heat-resistant resin is disadvantageous in that high heat treating temperatures could not be employed and productivity could not be increased accordingly.
In the light of these circumstances, PMMA which has low heat resistance withstands by no means high temperatures during the production of optical discs or on use. It has been hence studied to use PC having higher heat resistance as a material of a transparent substrate. However, PC is not always recognized as sufficient in heat resistance, and it has been demanded to develop a resin material having improved heat resistance.
A transparent resin consisting mainly of carbon and hydrogen (hydrocarbon resins) and having a high softening point has been proposed as a material free from the disadvantages associated with the conventional resins, e.g., PC and PMMA, as disclosed, e.g., in JP-A-63-43910 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
However, hydrocarbon resins having high softening points have poor adhesion to a recording layer, as is usual with polyolefin resins, and fail to assure a sufficient life of an optical disc.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a substrate of optical discs excellent in adhesion to a recording layer formed thereon.
Other objects and effects of the present invention will be apparent from the following description.
The present inventors have extensively studied on improvement of adhesion of hydrocarbon resins to a recording layer and, as a result, found that adhesion to a recording layer can be markedly improved by treating the surface of a molded article with a fluorine-containing gas, thus having reached the present invention.
The present invention provides a substrate for an optical disc which comprises a transparent hydrocarbon resin having a softening point of not less than 100.degree. C., the surface thereof having been contact treated with a fluorine-containing gas so that the treated surface has a contact angle with water of not more than 90.degree. and the weight gain due to the treatment does not exceed 1% by weight.